Why does the skin on your fingers and toes wrinkle after immersion in water? A panselectionist answer.

June 22, 2022 • 9:45 am

All of us have noticed that after a period of immersion in water, the skin on both our fingers and toes wrinkles up, but not the skin anywhere else on our body. Here are two photos of the crenulated digits:

From The Conversation


This raises two questions:

a.) What is the mechanism for the wrinkling?

b.) Is there any usefulness or “adaptive significance” of the wrinkling? That is, did natural selection favor it because the wrinkles are useful. 

The two articles below, the first a new popular summary from the BBC and the second a year-old scientific paper discussing the “adaptive significance” of the wrinkling, suggest answers to both questions.

It turns out that we know the mechanism of wrinkling pretty well, but, despite the assurance of both articles, we still have no idea whether it’s an “adaptive” response to water or merely some epiphenomenon that makes no difference to our well being or reproductive output.  That both articles immediately look for an adaptive “reason” why natural selection promoted finger and toe wrinkling is an example of what Steve Gould called “naive pan-selectionism”: assuming that every feature has natural selection behind the evolution of that feature, and favoring the production of that feature—in this case, wrinkling.

Panselectionists often accept pretty scanty evidence as being supportive of their theory, and I think you can see that here.

Click on both screenshots to read the article; the pdf of the scientific article (in PLOS One; reference at bottom) can be downloaded here.



I’ll use facts from both articles, but quotes will be attributed to one or the other.

First, how long does it take to wrinkle up? It depends on the temperature, with 3.5 minutes in warm water to begin wrinkling (40º C or 104° F) and 10 minutes in tepid water (20º C or 68° F). But even in cool water we will wrinkle.

How does it happen? Scientists first thought that it was simple osmosis: the skin cells absorbed ambient water and that made the cells swell up, causing wrinkles. But then they noticed that if the median nerve in the arm is severed, there is no wrinkling. That rules out the osmosis theory as a complete explanation. Osmosis may contribute a bit to the wrinkling, but nerves and blood vessels are also involved. Author Davis of the PLOS ONE paper says this:

Explanations for the wrinkling of the skin include a passive response of the skin to immersion, or an active process that creates the wrinkles for a functional purpose. There is overwhelming evidence that finger-wrinkling is an active process. The small blood vessels of the fingertip constrict, which creates valleys in the skin surface, triggered by water entering sweat pores . Note that a passive explanation would usually assume that water absorbs into the skin, pushing up ridges. This vasoconstriction appears to occur most readily at a temperature of around 40° Celsius, or the temperature of a warm bath [2]. People with autonomic neurological conditions including Parkinson’s, cystic fibrosis, congestive heart failure or diabetic neuropathy may show abnormal or asymmetric wrinkling in the affected parts of the body.

Note that in the first sentence he conflates an “active process” with “an adaptation that has a functional purpose.” This isn’t necessarily true. We get wrinkles, gray hair, and liver spots with age, which are “active processes,” but that doesn’t mean those features are the direct products of natural selection. (What is the adaptive function of liver spots?). The BBC adds a bit more about the mechanism:

Wilder-Smith and his colleagues proposed that when our hands are immersed in water, the sweat ducts in our fingers open up to allow water in, which leads to an imbalance in the salts in our skin. This change in the salt balance triggers the firing of nerve fibres in the fingers, leading to the blood vessels around the sweat ducts to constrict. This in turn causes a loss of volume in the fleshy area of the fingertip, which pulls the overlying skin downwards so that it distorts into wrinkles. The pattern of the wrinkles depends on the way the outermost layer of skin – the epidermis – is anchored to the layers beneath it.

The involvement of nerves explains why some conditions that affect nerves (see first indented para above) affect skin wrinkling.

Let’s assume, then, that we have a pretty good idea of how this happens in fingers, though nobody says much about toes or the rest of the body. (Toes are also sorely neglected in the “adaptive” explanation.

Both sets of authors then set about explaining why natural selection would favor such wrinkling (again, they discuss only fingers, not toes). The experiment describe in the second link above, which gives results in line with previous studies, suggests that the wrinkled skin allows you to grab wet objects with more force than if your skin is unwrinkled and wet. And if your fingers are wrinkled, you’re likely to be in an environment where there are wet objects.  The purported mechanism for this is the same one for treads and valleys in tires: the “channels” in our finger wrinkles suposedly help squeeze out the water when we’re gripping a wet object, allowing better contact with the object. (But what about the toes?)

Davis, then, did a study estimating the strength it took to grip a small and initially DRY plastic disk under three conditions:

a. dry unwrinkled fingers

b. wet wrinkled fingers (note: they apparently didn’t use dry wrinkled fingers, but it’s not clear from the paper. In fact, if they used dry unwrinkled fingers, it would make the adaptive explanation less credible.)

c. wet unwrinkled fingers

Not did they use wet objects, which is crucial for their adaptive hypothesis, though of course gripping a plastic disk with wet wrinkled fingers will make the object wet. Note also that the object is small and light (the BBC says it weighed as much as a couple of coins).

I won’t go into the detail to measure force, but they had an apparatus that measured both grip strength and the ability of the subject to lift up the object and hold it sufficiently tightly so it could be manipulated to follow a computer track. Here’s a photo from the paper:

(From paper): Fig 1. Picture of the equipment in use. The participant is gripping a load cell between finger and thumb. The participant’s task is to pull up on the second load cell to match a force trace that appears on the laptop monitor. The current load force is shown as a red circle, and the history of the participant’s force is shown as a trail of green dots.

The results: people with wet wrinkled fingers and those with dry fingers had similar grip forces, but those with wet, unwrinkled fingers needed significantly more force to grip the disk. Here’s one graph (just look at the top three lines) showing no significant difference between wrinkled-finger force (red) and dry-fingered force (purple), but significantly more force needed using wet, unwrinkled fingers. (The paper give statistics). This shows no real benefit of wet, wrinkled fingers over dry fingers when gripping the disk, but if your fingers are wet and unwrinkled, it’s harder to grip (the plastic get slippery).

(From paper): Fig 2. Comparison of performance across conditions. Mean grip force (thinner traces) and load force (thicker traces) when participants tracked a load weight target (black line). Participants with wrinkled fingers produced a grip force that did not differ from that used by people with dry fingers in the static hold phase, however people with wet but non-wrinkly fingers used a significantly higher amount of grip. The shaded area indicates the pointwise ±1 standard error of each mean trace. Lines below the trace indicate the attack phase (A) of the trial, the static phase (S) and the decay phase (D).

Here’s another graph that shows pretty much the same thing, but showing the grip force needed to sustain the load of the plastic disk under the same three conditions but with varying “load force” (weight, which could be manipulated). Green is wet, unwrinkled fingers, red is wet, wrinkled fingers, and blue is dry unwrinkled (normal) fingers:

(From paper): Fig 4. Relationship between grip and load force in Dry, Wet and Wrinkly conditions. This illustrates the grand mean of the grip and load forces for the whole duration of the trail, minus the first 1000 ms. The target force is shown as a dashed line. The three grip force traces lie above this line, indicating the safety margin. The ‘easiest’ condition, Dry (blue trace) follows the target force most closely. The ‘hardest’, Wet (green trace), shows a higher safety margin, and looser coordination. Participants with Wrinkly fingers (red trace) lie between the two.

Wet unwrinkled fingers require more force to hold the disk than do dry ones. Wet, wrinkled fingers aren’t superior to either, but intermediate between them. (No statistics are given, but another graph implies that none of the differences between the lines in the plot right above are significant.)

The overall conclusion is not strong. Clearly, wet unwrinkled fingers make it harder to grip a smooth plastic object than either dry fingers or wet wrinkled fingers (DUH), but wet wrinkled fingers don’t make it easier to grasp an object than dry unwrinkled fingers. In other words, any advantage of wrinkling is only when it’s compared to wet unwrinkled fingers. Otherwise, dry fingers grasping a dry object are marginally (and nonsignificantly) better than wet, wrinkled fingers.

What can you conclude from this? I’d say, “not much”, but the author of both the BBC article and of the paper seem to think that wrinkling is an adaptation that evolved in our ancestors to enable them to grip objects under wet conditions:


This suggests that humans may have evolved fingertip and toe wrinkling at some point in our past to help us grip wet objects and surfaces.

“Since it seems to give better grip under water, I would assume that it has to do with either locomotion in very wet conditions or potentially with manipulating objects under water,” says Tom Smulders, an evolutionary neuroscientist at Newcastle University who led the 2013 study. It could have given our ancestors a key advantage when it came to walking over wet rocks or gripping branches, for example. Alternatively, it could have helped us when catching or foraging for food such as shellfish.

From the paper:

Grip and load force coordination is an important aspect of object handling. The ability to generate the correct amount of grip force for a given load means that the minimum necessary amount of energy is used by the muscles that control the fingers and hands, and means that objects are less likely to be dropped or to be crushed. Efficient grip force coordination is seen in many extant primates, and is likely to have evolved early in the primate lineage [13]. The grip force required to stabilise a wet object is greater than the force required for a dry object, since the coefficient of friction of the digit-object interface is reduced [8]. It would therefore benefit an animal to gain an advantage in handling wet objects, as this would increase success in hunting and foraging in watery environments. The skin of the fingertip is already adapted for regulation of moisture at the contact surface [14]. Fingertip wrinkles would seem to afford an enhanced advantage in object handling, and may plausibly aid travel and clambering in wet areas, especially if combined with wrinkled toes.

Ergo, it helped us “hunt and forage in watery environments.” But this raises a number of questions:

a.) If you’re hunting or foraging in a watery environment, but your hands have been immersed for fewer than 20 minutes so they’re unwrinkled, you’re better off gripping a dry object with dry hands instead of wet ones. You have an advantage with wrinkled fingers only if they’ve been underwater long enough to get wrinkled, and that advantage is only over unwrinkled wet fingers so long as you’re gripping an object that is itself wet, like a plastic disk that your fingers have wetted.  If you’re trying to grab a dry object when your hands are wet and wrinkled, you’re worse off than when using dry hands.

b.) They did not test the three conditions when gripping large dry objects like a tree branch or an animal, which may not behave like plastic disks! This is essential if you think that either grabbing dry objects was important for our ancestors even when our fingers were wrinkled from having been immersed in water.

b.) We did not evolve in a watery environment; the “aquatic ape” hypothesis has long been dispelled. As for our relatives, the BBC article says “only one other primate has so far been found to have water-induced wrinkling of the fingers—Japanese macaques.” (Naturally, they show a photo of a macaque sitting in water.) I’m not sure if other primates have eve been tested (no such tests are referenced), but if chimps, bonobos, and orangs show finger wrinkling, that would imply that it did NOT evolve to enhance grip strength in watery environments. These primates don’t live in those environments!

d) What about doing the study with dry wrinkled fingers? (You quickly dry them before grasping the object.) The adaptive hypothesis would predict that there would be no grasping advantage of dry wrinkled fingers over dry unwrinkled fingers. They didn’t do that experiment (as far as I can see).

e.) What about the TOES? They get wrinkled too. The paper posit that wrinkled toes would aid “travel and clambering in wet areas”, but that is pure speculation—not even a hypothesis. It could be fairly easily tested, but wasn’t.

f.) If wrinkly skin is pretty much as good as dry skin for gripping almost anything, why don’t we have permanently wrinkled skin? Author Davis has an answer:

A previous study of object manipulation with wrinkled fingers found that wet objects were moved more quickly when the fingers were wrinkly compared to dry [15]. Importantly, there is no difference in tactile sensitivity in wrinkled fingers compared to dry [16], meaning that people are not trading off acuity for friction at the fingertip. It is therefore reasonable to wonder why healthy people do not have permanently wrinkled fingers. The answer presumably lies in the changes in the mechanical properties of the finger tissues, where there may be lower shear resistance when the finger is wrinkled [17]. Previous studies have also suggested differences in manipulation across the lifespan [1820]; the present results show age-related effects, although they are rather weak in this sample. Our journey through life leads us to develop strategies for handling familiar and unfamiliar objects, so it seems likely that strategic changes, along with sensory and motor changes, will affect how children and adults perform tasks with handheld objects [21].

Here we have ultimate pan-selectionism: if your hypothesis fails to explain another phenomenon, you simply make up a reason why that’s also adaptive. In this case, Davis posits “lower shear resistance” for wrinkled fingers, which for a reason he fails to specify must confer a disadvantage (presumably because you can’t hold onto an object as tightly).

I’m not at all convinced by this explanation or the supporting data, as they’re contradicted by evolutionary observations and by the absence of data on wrinkled toes. As the BBC says, some believe that wrinkling “could just be a coincidental physiological response with no adaptive function.” (Go have a look at that link!). I am one of those skeptics. What surprises me is that that statement is the sing caveat (and doesn’t reprise what’s at the link) in a whole article pushing the “adaptive wrinkling in wet environments” hypothesis.

Other venues have also picked up this result, and I guess they are either overly credulous or didn’t read the paper carefully enough. Or they didn’t ask probing questions.

h/t: Peter


Davis NJ (2021) Water-immersion finger-wrinkling improves grip efficiency in handling wet objects. PLOS ONE 16(7): e0253185. https://doi.org/10.1371/journal.pone.0253185

How Asian honeybees kill their fearsome hornet predators

May 10, 2022 • 12:45 pm

I can’t remember why I opened the natural-selection chapter in Why Evolution is True (chapter 5: “The Engine of Evolution”) with the story of the Asian giant hornet (Vespa mandarina) and of the counterdefense of its prey of native honeybees. (The European honeybee, more recently introduced into Asia, has not evolved such a bizarre and amazing defense.)  The giant hornet is much to be feared by both honeybees and humans: it’s as big as your thumb, and several humans (and millions of bees) die from its attacks every year.

Since you all should have a copy of WEIT (as Hitch would say, “Available at fine bookstores everywhere”), I won’t recount the story of the native honeybees’ defense, but it involve luring the voracious hornet scout into the bee nest and the cooking it to death:  surrounding it with a ball of vibrating bee bodies that raises the ball’s internal temperature to 117 degrees F (47°C): a temperature that kills the wasp but not the bees.

I suppose I put that story in because it’s a stunning example of the power of natural selection to shape behavior (in both wasp and bee), and not many people knew about it. Now I hear that a lot of readers especially liked that story. It is a true one, and in this segment from BBC Earth, you can see the nefarious hornet scout discovering a hive of native honeybees.

The scout marks the hive with pheromones and usually flies back to recruit a swarm of fellow hornets to return to the nest to destroy it: a process that can take only a short while as the wasps  nest in minutes, decapitate adult bees and steal their honey and and bee grubs. But, as I relate in the book, sometimes, as here, the hornet scout never gets back to its own nest because of the counterdefense. The native bees lure it inside and cover it with vibrating bees that kill it.

This video is, of course, far more vivid that what I could say in words, so I want to show it here. But imagine the sequence of evolutionary steps that produced this defense!

If you want to see how these hornets slaughter the non-native European honeybees, watch this gruesome attack (each wasp can kill 40 bees per minute!). I’m sure I’ve shown this video somewhere in the distant past.

Now if you’ll excuse me, i’ll go home to rest.

Ivory poaching imposing selection on elephants to evolve shorter tusks

October 24, 2021 • 9:30 am

Here we have a case of selection by humans—killing elephants that have tusks because ivory is so valuable—increasing the frequency of tuskless African elephants in Mozambique over a 28-year period. (As we’ll see, only the proportion of tuskless females increased.)  We have similar examples from other species, as in the reduction of horn size in bighorn sheep hunted for their horns as trophies, and the reduction in the size of some fish due to commercial fisherman going after the big ones.

Is this artificial or natural selection? Well, you could say it’s artificial selection because humans are doing the choosing, but after all human are part of nature. And this selection was not conducted to arrive at a given end. Dachshunds were selected to look like hot dogs to root out badgers in their burrows, but the reduction of tusk size in elephant, or horns in sheep, was not a deliberate target of selection, but a byproduct of greed. So I would hesitate to characterize this as artificial selection, since it’s not like breeders choosing a given characteristic to effect a desired change. In fact, the evolutionary change that occurred is the opposite of what the “selectors” wanted.

You can find the article in Science by clicking on the screenshot below, or get the pdf here.  There’s a two page shorter take that’s an easier read, “Of war, tusks, and genes,” here.

The phenomenon: a civil war in Mozambique from 1977 to 1992, which increased the frequency of tuskless female elephants from 18.5% to 50.9%, nearly a threefold increase. Why? A model showed that such a change (which occurs among generations, so it’s not just selective killing within a generation) must have been due to natural selection rather than genetic drift. The killing was motivated by a desire to get money to fund the conflict.  A female without tusks had five times the chance of surviving as a tusked female. That imposed strong selection in favor of tuskless females.

Usually, tuskless elephants are at a disadvantage, for tusks are multi-use features, employed for defense, digging holes for water, male-male competition, and stripping bark from trees to get food. But the natural selection to keep tusks in females was weaker than the “artificial selection” by humans against tusks.

Here’s a photo of a tuskless vs. a tusked female:

Photo by Finbarr O’Reilly for The New York Times

And the only kind of male that we see: ones with big tusks (tusk size varies, of course, as they continue to grow as the elephant lives). Tusks are homologous with our incisor teeth.

The authors first tried to determine the genetic basis of having versus lacking tusks. It turns out that, by and large, tusklessness behaves not as a complex trait caused by changes in many genes of small effect, but as a single dominant mutation on the X chromosome (like us, elephant males are XY and females are XX). Further, the dominant mutation causing tusklessness is lethal in males, killing them before birth. (This is probably not because the tuskless gene form is itself lethal, but is closely linked to a gene that is a recessive lethal.)

So here are the “genotypes” of the elephants. I’ve used “x” as the gene form on the X chromosome that produces tusks, and “X” as the alternative dominant allele that makes you tuskless.

Males: All have tusks and are thus xY. (Males have only one X chromosome and also a Y.) The XY genotype is lethal, so we never see males carrying the tuskless gene form (XY). Ergo, there are no tuskless males.

Females: We see two types:

Tuskless: Xx. These females will lose half their male offspring because when mated to an xy male (the only viable type), they produce half xY males, which are tuskers, and half XY males, which are lethal. Thus a population of tuskless females will produce a sex ratio in their offspring skewed towards females, which is what is observed.

We never see XX tuskless females because they’d have to inherit one “X” from from their fathers, but that XY genotype is lethal.

With tusks: xx.

There are a few complications, as other genes are involved (for example tusked mothers, who are xx, produce only 91% of tusked daughters when you’d expect the xx by xY cross to produce 100% xx (tusked) daughters. So things are not quite so simple, but in general a single gene seems largely responsible for the tuskless condition. (You might expect this, because if many genes were involved you simply wouldn’t get females lacking tusks: you’d get females with slightly smaller tusks, who would still be killed for their ivory. It would thus take many generations instead of a couple to raise the frequency of tuskless females.)

I won’t go into the gory genetic details, but the authors sequenced entire genomes from tusked and tuskless males and females and looked for signs of natural selection on some genes, comparing the tusked versus tuskless females. (One sign of rapid selection for tusklessness, for the cognoscenti, is the presence of DNA bases recurrent and common near the gene causing tusklessness.)

The researchers found one X-linked gene form with strong signs of selection called AMELX, which in other mammals codes for a protein that leads to the mineralization of enamel and regulates other tooth-associated genes. Another gene not on the sex chromosome, MEP1a, also is associated with tusklessness, but not as strongly. This gene, too, is known to be associated with tooth formation in other mammals. Here’s the diagram from the paper of which parts of the tusk are controlled by which gene. You can see that AMELX is expressed only in the “tusky” part of the tusk:

(From paper): Putative functional effects of candidate loci on tusk morphology.A cross section of an African elephant tusk shows the anatomical position of (a) enamel, (b) cementum, (c) dentin (ivory), (d) periodontium, and (e) root of the tusk. Dark blue circles indicate regions known or proposed to be affected by candidate gene AMELX. Light blue circles are proposed to be affected by candidate gene MEP1a. Neither gene is known to affect the formation of the dental pulp (black interior of cross section).

The upshot: Human-imposed (“anthropogenic”) selection that causes evolution in the wild has been demonstrated before, so this phenomenon is not new. What is new is that the genes involved in an anthropogenic evolutionary change—the increase in frequency of the tuskless allele, which is evolution—have been identified for the first time, and we know the kind of selection that’s caused the evolution. What is also unusual (I know of no other case) is that selection for tusklessness is in opposite directions (“antagonistic selection”) in the two sexes so long as tuskless females survive better. As the authors note:

Physical linkage between AMELX and proximate male-lethal loci on the X chromosome, such as HCCS, may underpin the proposed X-linked dominant, male-lethal inheritance of tusklessness in the Gorongosa population. If our interpretation is correct, this study represents a rare example of human-mediated selection favoring a female-specific trait despite its previously unknown deleterious effect in males (sexually antagonistic selection). Given the timeframe of selection, speed of evolutionary response, and known presence of the selected phenotype before the selective event, the selection of standing genetic variation at these loci is the most plausible explanation for the rapid rise of tusklessness during this 15-year period of conflict.

What of the future? Even though the conflict is over, poachers continue to kill tuskers for their ivory in much of Africa. What will happen? We expect the frequency of the dominant tuskless allele to increase. That itself will not lead to extinction of the population because tuskless males are simply not produced: all tuskless females will remain Xx and produce half the normal number of males. Tusked females will still be produced as Xx females crossed to xY males will produce both Xx (tuskless) and xx (tusked) females.  But the reduction in the number of males produced by anthropogenic selection, coupled with continual poaching of both males and females with tusks may drive the population size so low, with an unequal sex ratio, that it could become severely endangered.

Since tusks are good for elephants, the solution is not only to ban the trade in ivory, which has been done in part, but some countries continue to trade in elephant ivory. Further, we must stop the poachers cold, as there’s still a market for both legal and illegal ivory, and prices are high. That’s easier said than done given the area that must be monitored. Note, though, that in 2017, Donald Trump lifted the ban on ivory imports from Zimbabwe, which had been put in place by his predecessor. And the elephant is the Republican symbol!

h/t: Pat, Matt, and several other readers.


S. C. Campbell-Staton et al.. 2021. Ivory poaching and the rapid evolution of tusklessness in elephantsScience 374, 483-487.

Sexual versus natural selection: a case in beetles

October 4, 2021 • 11:15 am

Although Darwin himself drew a bit of a distinction between natural and sexual selection, the latter is really a special case of the former. Sexual selection is simply natural selection among individuals for their ability to acquire a mate: one of many behaviors that determine how many genes you leave behind. And there are cases in which it’s hard to determine which form of selection is going on. If a male’s sperm swim faster than the sperm of other males in a species where females are multiply inseminated (e.g., fruit flies), is that male experiencing positive natural selection or positive sexual selection?

Well, the details don’t matter so long as we keep track of what’s going on. In a new paper in Nature Communications, also summarized in a short News and Views in Current Biology, a group of investigators demonstrate how sexual selection can conflict with other forms of natural selection. The experiment was hard and laborious, but the results can be conveyed simply, and I’ll try.

I’d suggest that if you read one of the two articles, it should be the second, as it’s shorter, written for a less specialized audience, but nevertheless an accurate summary. But if you want the original paper, click on the screenshot below or get the pdf here.

To read the Current Biology precis, click on the screenshot below or get the pdf here. 

We begin with a sexually dimorphic beetle (below), Gnatocerus comutus, the “broad-horned flour beetle” that’s a pest in grain silos.  As you see, it’s sexually dimorphic, with males having bigger heads and, notably, a huge pair of mandibles (arrows). The females lack mandibles. That’s a hint that the mandibles aren’t used for defense against predators or for predation, but are used in male-male competition for females (if they helped procure prey or fight off predators, the females should have them, too). And indeed, that’s exactly what the mandibles are used for.


A prediction from this difference is that there is a metabolic cost to growing those mandibles, and although males with mandibles have higher overall fitness, if you could remove male-male competition, the mandibles wouldn’t give you a selective advantage. In that case they would be selected to evolve a smaller size as the resources used to grow them could be directed at other aspects of fitness. Every time you see a case of sexual dimorphism involving a cumbersome or conspicuous trait, you can predict that that trait has a cost, and is involved in sexual selection (the male peacock’s tail is the classic example).

The authors of the first paper did a clever experiment. Instead of removing male-male competition (you could do this by pairing one male with one female for generations; I predict the mandibles would get smaller), they exposed the males and females (separately) to a vicious predator, the assassin bug Amphibolus venator, which doesn’t regularly prey on G. comutus in nature but will eat anything it encounters.

Here’s the assassin bug confronting its potential prey (from the Current Biology paper):

What happened?

First, over 7 generations, with the males who escaped predation mated to control (unselected) females, the offspring of the escaping males evolved a smaller size. Clearly they weren’t defending themselves against predation from the assassin bugs; rather, the mandibles appear to have been an impediment to survival. The authors suggest that they’re heavy and impede the mobility you need to escape predators.

And, as expected, those small-jawed males whose descendants survived 7 generations of predation lost out when allowed to compete with regular males for females: they won contests only half as often as males from control treatments or female-only predation treatments. Jaws matter at mating time!

What was not expected was that the female descendants of the predated males actually got fitter.  Why? Because their abdomens got larger, possibly enabling them to produce more eggs. (An alternative is that females’ sperm storage organs got larger, enabling them to store more sperm.) But why would this happen? Probably because there is a genetic correlation between male mandible size and, in females, either abdomen or sperm-storage organ size, so if you make the former smaller, the latter get bigger. There’s independent evidence for this. (We don’t know about the developmental pathways that connect male jaws and female abdomens.)

What this shows is not only the cost of sexual selection, but a cost that’s levied in both males and females. If there were no male-male competition, and males had small mandibles, females would leave more offspring.  You might ask, then, given that there are of evolving mandibles paid by both sexes, why do males still evolve large jaws?

The answer must be that the genes that increased male mandible size in the past still had a NET advantage over genes for smaller mandibles. In other words, their cost in reduced ability to escape predators and reduced female offspring number was more than offset by the advantage of winning contests for females. This shows that fitness increases in one sex (the larger mandibles that evolved in males) can be paid for by fitness reductions in the other sex as well (reduced reproductive output of smaller-bellied females).

And so Nature has woven a tangled web here, but one somewhat untangled by the tedious but revealing experiments of the researchers who wrote the first paper.

Sequencing of penguin genes gives family tree, presumed geographic origin, and hints about natural selection

August 20, 2020 • 9:00 am

A big group of researchers from around the world—science is truly international in this case—just published a paper in Proceedings of the National Academy of Sciences that involved sequencing the complete genome of 18 species of penguins as well as an outgroup, the southern giant petrel.  (Researchers differ on the number of extant penguin species, ranging from 17 to 20, as some populations are geographically isolated, making it hard to discern species status.)

The DNA information was combined with fossil data to yield a family tree of the living species, and also to reconstruct their evolutionary history, which suggested that the ancestor of all living and fossil penguins probably lived not in Antarctica, but on the coasts of Australia and/or New Zealand. Finally, the researchers were able to narrow in on a group of genes that may have undergone natural selection in the group, suggesting which adaptations were crucial for making a well-functioning penguin.

You can access the paper by clicking on the screenshot below, or see the pdf here. The full reference is at the bottom, and there’s a popular summary article at CNN.

I’ll try to be brief here. First, I’ve put below the family tree of living penguins deduced from the DNA information, with the divergence times that come from both DNA and fossil data. The radiation started around the beginning of the Miocene, roughly 22 million years ago.

As you can see, the largest species—the emperor and king penguins, form their own “outgroup” to the rest of the penguins, splitting off from the rest early in the group’s radiation but splitting from each other only about two million years ago. (Despite the radiation being old, most modern species split from their closest relatives only within the last few million years.)

The average temperature of the southern ocean is given by the graph in white and the scale on the left, with the dotted red line showing the beginning of the “strengthening” of the Antarctic Circumpolar Current (ACC), a strong ocean current that sweeps clockwise around Antarctica as seen from the South Pole, isolating the continent from warmer ocean temperatures to the north and allowing the ice sheet to persist.  A lot of the radiation followed the advent of this current’s new strength, which also coincided with the opening of the Drake Passage, creating a water gap between the previously connected land masses of Antarctica and South America.  It also produced a lot of sub-Antarctic islands that were also sites for colonization. And geographic isolation, possibly enforced by temperature, is an impetus for the formation of new species.

It was this stronger current and geographic separation that, the authors say, prompted new speciation events in penguins (most biologists assume that new species usually arise after populations become geographically separated). They did, however, detect some gene flow between penguin species, though it wasn’t extensive enough to wipe out the differences that produced this tree:

Using some assumptions and a complicated program, the authors could use the phylogeny to estimate the geographic range of the ancestral species as well as the ranges of ancestors within the phylogeny. Those are indicated with the letters A through I in the figure above.

The procedure is complicated, but it’s done the way evolutionists estimate ancestral traits of species—assuming that ancestors pass traits down to their descendants. In this case “geographic range” is considered a trait of a species. For example, if two closely related but distinct species occupy geographic areas that are close together, one can assume that their joint ancestor lived in that general area as well. The figure below shows the geographic areas that correspond the the letters of existing penguins (under their names) as well as the ancestors of groups (letters at the branch points).

The range of the ancestral node is letter I, and you can see that corresponds to the coastal areas of Australia and New Zealand, which, the authors assume, is where the ancestral species that gave rise to all modern penguins lived. This is a big conclusion of the paper, but since there are numerous assumptions that go into the biogeographic model, and not a lot of fossil data, I would take that conclusion as very tentative. If it’s true, that means that penguins evolved in areas where the water temperature at the time was abut 9ºC (48° F), and then some descendants (e.g. kings and adelies) colonized colder waters, while others (e.g.. Galápagos and African penguins) colonized warmer waters.

The ancestor of king and emperor penguins presumably lived on the coast of South America or Antarctica (letters A and C); kings currently breed on subantarctic islands and emperors only in Antarctica.

It’s possible, looking at the amount of genetic variation within whole genomes, to discern something about the demographic history (i.e., population sizes) of penguin species (again, there are some big assumptions here). You see below the plot of the “effective population size” (a figure that’s usually somewhat lower than the actual census size) for six species of penguins. Most show a strong drop in population size between about 70,000 and 40,000 years ago, which corresponds to the last glacial maximum (LGM, indicated by the vertical line). The authors say that the extreme cold during the LGM may have reduced the productivity of marine waters, and hence the abundance of fish and krill, the main diet of penguins.  That, in turn, is said to have reduced the population size of many penguin species:

Finally, there are ways to detect genes in a lineage that may have been subject to natural selection. This is done by finding genes in which there is an elevated rate of amino acid substitutions, which change the structure of a protein, over the rate of presumed “neutral” changes in DNA, which don’t change protein structure.  The assumption here, which is a good one, is that a relatively faster rate of protein evolution was promoted by natural selection.

Here’s a diagram of some of the genes, and classes of genes, that, says the analysis, underwent (positive) natural selection, presumably conferring adaptation on individuals in the various species. The genes that apparently evolved adaptively are in pathways influencing thermoregulation, osmoregulation via renal function (fluid and salt balance), blood pressure regulation (helps conserve oxygen and maintain core body temperatures), and oxygenation (important in deep diving). Some of the genes are named in the diagram below. Again, these genes are identified as candidates for adaptation only from their pattern of DNA substitution in the tree, and we don’t know for sure whether the changes really were adaptive, much less how they affected the animal.

The authors conclude on a sad note, saying that it took penguins millions of years to adapt to new temperatures (including colonizing the relatively warm waters around the Galápagos Islands), and thus would likely be unable to adapt to the relatively fast temperature increases accompanying global warming. While one would think that a history of slow adaptation doesn’t say anything about how fast adaptation could proceed under more rapid environmental change, we already know that global warming is seriously damaging some populations of penguins. The CNN report quotes the first author of the paper and describes some heartbreaking changes:

“Right now, changes in the climate and environment are going too fast for some species to respond to the climate change,” said Juliana Vianna, associate professor at the Pontifical Catholic University of Chile, in the UC Berkeley statement.

The different elements of climate change culminate in a perfect storm. Disappearing sea ice mean fewer breeding and resting grounds for emperor penguins. The reduced ice and warming oceans also mean less krill, the main component of the penguins’ diet.

The world’s second-largest emperor penguin colony has almost disappeared; thousands of emperor penguin chicks in Antarctica drowned when sea ice was destroyed by storms in 2016. Reoccuring storms in 2017 and 2018 led to the death of almost all the chicks at the site each season.

Some penguin colonies in the Antarctic have declined by more than 75% over the past 50 years, largely as a result of climate change.

In the Galapagos, penguin populations are declining as warm El Nino events — a weather phenomenon that sees warming of the eastern Pacific Ocean — happen more frequently and with greater severity. In Africa, warming waters off the southern coast have also caused penguin populations to drop drastically.

I’m lucky to have seen five species of penguins, including kings, on my trip to Antarctica last winter. It would break my heart if we humans, through our depredation of the environment, drove these magnificent products of evolution to extinction. They were here long before we were!

h/t: Matthew, Terrance


Vianna, J. A., F. A. N. Fernandes, M. J. Frugone, H. V. Figueiró, L. R. Pertierra, D. Noll, K. Bi, C. Y. Wang-Claypool, A. Lowther, P. Parker, C. Le Bohec, F. Bonadonna, B. Wienecke, P. Pistorius, A. Steinfurth, C. P. Burridge, G. P. M. Dantas, E. Poulin, W. B. Simison, J. Henderson, E. Eizirik, M. F. Nery, and R. C. K. Bowie. 2020. Genome-wide analyses reveal drivers of penguin diversification. Proceedings of the National Academy of Sciences:202006659.

A stunning case of mimicry

January 21, 2020 • 9:00 am

I don’t remember encountering this case of mimicry, but it’s so amazing that, when I became aware of it from a tweet (yes, Twitter has its uses), I decided to give it a post of its own.

First the tweet, sent to me by Matthew. He added, “This is the Iranian viper, as featured in Seven Worlds, One Planet, made by the BBC. Amazing.”

You don’t need to translate the Spanish, though, as the video below tells all. I swear that when I first watched it, I thought there was a real spider crawling on the snake’s back.

The snake is the spider-tailed horned viper, Pseudocerastes urarachnoides, which has a small range in Western Iran (map from Wikipedia):

It wasn’t described as a new species until 2006 in the paper below (free access); before that it was thought to be the already-describe Persian horned viper. (I guess they overlooked the tail ornament.)

Here’s a photo of the tail “spider” from the paper; the one below that is from Wikipedia. The resemblance may not be precise, but (as you see above), when the ornament is moved about, it looks remarkably like a spider—certainly good enough to fool birds.

In that paper, the authors didn’t know how the tail ornament was used, but were impressed at its spider-like appearance. And they guessed accurately:

This raises the question of the elaborate and sophisticated appearance of the caudal appendage in our new species, as the waving or wriggling motion of a distinctively colored tail tip seems perfectly adequate to attract lizard and anuran prey. We can only speculate that in the case of the present species, the caudal lure serves to deceive a more specific kind of prey, such as shrews or birds. Indeed, ZMGU 1300 [the specimen number] contains an undigested, unidentified passerine bird in the stomach (the feet protruding through the body wall).

Only later, using live captive specimens, did researchers see that the ornament did indeed attract birds that the snake caught and consumed, as in the video above.

Any biologist who sees this is immediately impressed by the ability of natural selection to mold not only morphology, but the behavior of the snake: the twitching of its tail so that the spider ornament appears to “walk.”  But any adaptation like this ornament must have incipient stages, and each subsequent modification must improve the adaptation—that is, it much give the snake possessing the “improved” improvement a reproductive advantage. (That advantage would derive from the better nutrition of a snake who caught more birds, and thus might have more offspring, increasing the proportion of genes for more spider-like ornaments.)

My own guess was that the ornament started with the simple twitching of the tail of an immobile snake, a twitching that might attract predators and, moreover, is already known in several snakes. After that, any mutation that modified the tail, making it look more like a spider, would give the snake a further reproductive advantage. And so we get the spider ornament, which might of course still be evolving. Concurrent with the evolution of the ornament itself would be the evolution of the snake’s tail-twitching behavior, which makes the caudal appendage resemble a spider nearly perfectly.

It turns out, of course, that I’m not the first person to think of this scenario. Discover Magazine wrote about this snake last spring, and speculated about its evolution:

“The evolution of luring is more complex than contrasting color or simple shaking — the movement is precisely adapted to duplicate prey movement frequencies, amplitudes and directions, at least in specialized cases.”It’s not uncommon for many snakes to do something similar with their tails to deceive prey. The common death adder of Australia buries itself in leaves, then writhes its tail like a worm to catch lizards and frogs. The Saharan sand viper conceals itself in sand with only its eyes and nostrils visible. When a lizard comes along, it sticks its tail out from the dirt, making it squirm like an insect larvae.  The behavior — and the elaborate body modifications that can accompany it — likely arose from a behavior common to many reptiles, Schwenk explains. When they are about to strike prey, any lizards and snakes enter a hyper-alert pose. The reptiles will focus their vision by cocking their heads to the side, arching their backs, and certain species will commonly vibrate their tail tip against the ground. This can distract the prey, which will shift its attention to the vibrating tail, ignoring the reptile mouth opening to grab them.“This simple pattern leads to selection causing refining of the tail form and motion to be more attractive to such prey by more accurately mimicking actual prey movements,” Schwenk theorizes. “The other ancestral condition that could have led to caudal luring, or possibly an intermediate step in the process, is the use of tail vibration for prey distraction rather than for luring.” Indeed, those most famous tail shakers, the rattlesnakes, sometimes also use caudal luring. For example, juvenile dusky pygmy rattlesnakes, whose rattle is so small it barely makes noise, wiggle their tails to attract prey. The behavior, in fact, may be key to how rattlesnakes evolved their distinctive rears, although this theory is somewhat controversial. “Like many other apparently simple things in biology, there is a lot of complexity to caudal luring that has barely been explored,” Schwenk says. “Much of this has been considered in a piecemeal fashion, but a thorough review and synthesis … has not been attempted.”

Now we’re not sure if this is the correct evolutionary pathway, but constructing a plausible step-by-step scenario like this, and showing that the intermediate “stages” occur as adaptations among existing species, is sufficient to refute the creationist claim that structures like the spider ornament could not have evolved and thus much have been created by God (or a “designer”, which means the same thing). The same kind of argument was used by Darwin in The Origin to refute Paley’s argument that the camera eye must have been created by God. Dawkins discusses it in the video below (and, as I recall, in his book The Blind Watchmaker).


Two biological puzzles

January 14, 2020 • 9:15 am

Here are two questions to ponder while I am doing other things today. The first comes from Matthew, whose words are indented:

Here’s a question which might be good to pose to readers.

Why are there no live-bearing birds? Live-birth has evolved many times in squamates, so is clearly within mutational reach of the reptilian genome (and interestingly, it generally leads to social behaviour). It has been argued that birds lay eggs because they would be too heavy to fly if they were carrying around young inside them. Apart from the obvious problem that bats manage fine, if that argument is right, you might expect some flightless birds to have been live-bearing. But they aren’t. Maybe they were in the past? Any hand-wavy explanations?


And I have my own question:

Why are there no herbivorous snakes?  There are lots of snakes in the world and they slither in the grass, but none of them eat it—or any other vegetation. They are all carnivores.

This is particuarly puzzling in light of the fact that the relatives of snakes—lizards—often eat a great deal of vegetation, and at least one species—the marine iguana of the Galápagos—eats only vegetation (algae; though rarely they’ll eat other stuff). So it is possible for reptiles to evolve into herbivores. (Many of the dinosaurs were plant-eaters.) Why haven’t snakes done it?

Neither Matthew and I know the answers here (after all, these questions bear on mutational possibility, evolutionary history, physiology, and so on), but the questions are interesting to ponder. They do show that not all conceivable “niches” get filled by evolution.

Here’s a nice video of a marine iguana (Amblyrhynchus cristatus) foraging; I saw many of these when I visited the Galápagos some years ago. It is also the only marine lizard. There are other marine reptiles like saltwater crocodiles, sea snakes, and of course marine turtles, but to my knowledge this is the only lizard that forages in the sea (they live mostly ashore).

Nathan Lents on the imperfection of the human body (it’s evolution, of course)

January 10, 2020 • 12:45 pm

UPDATE:  I found out that the well-known evolutionary geneticist John C. Avise published a related book in 2010, but one that concentrates on a different line of evidence for evolution. John’s book (screenshot of cover below with link to Amazon) lays out the many suboptimal features of the human genome. He thus concentrates on molecular evidence, noting the many features in that bailiwick whose imperfection gives evidence for evolution and against intelligent design.  Lents’s and Avise’s books thus make a good pair, since the former seems to deal mostly with anatomy and physiology and the latter with molecular data. I’ll be reading both of them.


Biologist Nathan Lents, whose abbreviated c.v. is given below, has been featured on this site before, both as a critic of creationism (good), but also as a defender of the Adam-and-Eve apologetics pushed by his religious friend Josh Swamidass (bad). But chalk up another two marks on Lents’s “good” side.  First, he’s written a book (click on screenshot below) that lays out all the suboptimal features of the human body—features whose imperfection gives evidence for evolution. I’m getting the book for teaching purposes, and here’s the Amazon summary:

Dating back to Darwin himself, the “argument from poor design” holds that examples of suboptimal structure/function demonstrate that nature does not have a designer. Perhaps surprisingly, human beings have more than our share of quirks and glitches. Besides speaking to our shared ancestry, these evolutionary “seams” reveal interesting things about our past. This offers a unique accounting of our evolutionary legacy and sheds new light on how to live in better harmony with our bodies, in all their flawed glory.

Nathan Lents is Professor of Biology at John Jay College and author of two recent books: Not So Different and Human Errors. With degrees in molecular biology and human physiology, and a postdoctoral fellowship in computational genomics, Lents tackles the evolution of human biology from a broad and interdisciplinary perspective. In addition to his research and teaching, he can be found defending sound evolutionary science in the pages of Science, Skeptic Magazine, the Wall Street Journal, The Guardian, and others.

And here’s a half-hour Center for Inquiry talk, clearly based on his book, in which Lents discusses how the flaws in the human body instantiate evolution. It’s not just that there are flaws—which support the notion that natural selection doesn’t produce absolute perfection, but simply the best result available given the existing genetic variation—but, more important: those flaws are understandable as the result of our evolution from ancestors who were different from us.

Some of Lents’s examples (like our broken gene in the Vitamin C synthesis pathway), are discussed in WEIT, but others, like the bizarre configuration of our nasal sinuses, aren’t. I haven’t seen the book, but it looks like a good compendium of evidence for evolution using something that everyone’s familiar with: the glitches and bugs in the human body.

It’s a good talk, and Lents is an energetic and lucid lecturer. I recommend that you listen to this, for you’ll learn stuff that will stay with you, and also serve to help you argue with creationists.

h/t: Michael

More about sexual selection in the New York Times

January 21, 2019 • 9:45 am

With the publication of his book The Evolution of Beauty (subtitle: How Darwin’s Forgotten Theory of Mate Choice Shapes the Animal World—and Us), Yale ornithologist Richard Prum gained an extraordinary amount of publicity in the popular press.  His theme was that “beauty”—that is, the evolution of extreme and stunning displays and ornamentation in male birds—results from a form of “runaway sexual selection” in which females’ random preference for extreme male traits produces amazing sexual dimorphism that has nothing to do with natural selection. (The peacock is perhaps the most famous example.) Prum’s book got two separate reviews in the New York Times, at least one other notice, and two big reportorial pieces, including recent the one below. The book was also nominated for a Pulitzer Prize for nonfiction, though it didn’t win.

Prum’s book is worth reading for two reasons. First, it presents a strong defense of the “runaway” model of sexual selection Prum calls it the “beauty happens” model, in which random female preferences lead to the exaggeration of male traits up to the point at which those traits actually hurt the male’s reproductive success (a peacock with a bigger tail would presumably not only be unable to fly, but would be a target for predators and find it hard to get around). Second some of Prum’s writing is very good, and his examples of exaggerated male behaviors and plumage engrossing and yet unknown to many laypeople.

But the book, as I’ve written before (see posts here), is tendentious. It ignores other models of sexual selection (except to denigrate them), it ignores the weaknesses of his own favored runaway model, and it misrepresents the views of evolutionary biologists (many of whom agree that the runaway may be important, but won’t buy into Prum’s view that it’s ubiquitous).  Prum claims that the runaway model is universally rejected by biologists in favor of “good genes” models (male traits indicate their genetic endowment). But that claim isn’t true: we just don’t have much data to distinguish all the competing models we have for how sexual selection works.

Further, Prum ties his model to progressive politics, saying that female choice in animals should hearten us because it shows that female “sexual autonomy” is natural. But such autonomy isn’t always present: many animals, for instance, have forced copulation. Bedbugs, for example, exhibit “traumatic insemination”, in which males bypass copulation by simply injecting sperm through the female body wall, with that sperm finding its way to the female eggs. Females don’t get to choose their mates, and copulation can actually kill them.

And there are many cases of forced and unwanted copulation by males, as well as male-male competition (viz., elephant seals) in which females are simply constrained to mate with whichever male wins a contest. Prum’s evocation of politics therefore demonstrates the “naturalistic fallacy”: that what happens in nature is what we should emulate. However, a lot of what happens in nature is stuff we shouldn’t emulate.

Prum also ties other models of sexual selection, including those in which a male’s traits indicate his vigor, health, or presence of “good genes”, to eugenics, and Nazi genocide, tarring the theories he doesn’t like with the social-justice cry of “Nazi”.  This is unconscionable. I can’t help but think, though, that Prum’s tying sexual selection to feminism was partly responsible for the book’s popularity and its Pulitzer nomination.

As I’ve written before, however, while Prum’s book received public approbation and good reviews—mostly from reviewers with no science background)—the reaction of the scientific community itself has been tepid and mostly critical for reasons I gave above. The three reviews I’ve read in scientific journals, including one by Gerald Borgia and Gregory Ball and another by Doug Futuyma, both highlight serious problem’s with Prum’s presentation, including the ignoring of alternative theories, the misrepresentation of the “beauty happens theory”, and the unwarranted connection between women’s rights and mate choice in birds. A more recent and much longer review, by Patricelli, Hebets, and Mendelson, published in Evolution (click on screenshot below for free access), was severely critical, and rightly so, though the authors did their best to be evenhanded and polite:

I’ve discussed this review before (full disclosure: I gave the authors some suggestions on a draft of their piece), and so won’t go over its contentions here. But if you want to read a review of Prum’s book—and one that is objective but critical—Patricelli et al. is the one to read. It is a good palliative for the publicity Prum gets repeatedly about his book.

That aside, several readers sent me the link to Ferris Jabr’s NYT piece above, suggesting that I write about it. I intended to, but was in Hawaii where I was having too much fun to work. Now that I’m back, I’ll summarize it as briefly as I can. (The piece is very long, and appeared in the NYT Sunday Magazine, an indication of how important the editors deemed the topic.)

Upshot:  Jabr’s piece is a mixed bag. (He’s a contributing writer to the New York Times and and often writes about science.)

The good bit is that Jabr at least indicates, as many writers haven’t, that the scientific community is lukewarm about The Evolution of Beauty and that Prum is somewhat dogmatic and dismissive of his critics. For example:

Despite his recent Pulitzer nomination, Prum still stings from the perceived scorn of his academic peers. But after speaking with numerous researchers in the field of sexual selection, I learned that all of Prum’s peers are well aware of his work and that many already accept some of the core tenets of his argument: namely that natural and sexual selection are distinct processes and that, in at least some cases, beauty reveals nothing about an individual’s health or vigor. At the same time, nearly every researcher I spoke to said that Prum inflates the importance of arbitrary preferences and Fisherian selection to the point of eclipsing all other possibilities. In conversation, Prum’s brilliance is obvious, but he has a tendency to be dogmatic, sometimes interrupting to dismiss an argument that does not agree with his own. Although he admits that certain forms of beauty may be linked to survival advantages, he does not seem particularly interested in engaging with the considerable research on this topic. When I asked him which studies he thought offered the strongest support of “good genes” and other benefits, he paused for a while before finally responding that it was not his job to review the literature.

Of course it was Prum’s job to review the literature, and especially to weigh his favored theory against alternatives, including “good genes” models and “sensory bias” models, in which female preference are not random but the byproduct of natural selection based on the species’ environment. How could it not be an author’s duty, when defending a theory, to review the literature for and against that theory?

Jabr also says this:

Like Darwin, Prum is so enchanted by the outcomes of aesthetic preferences that he mostly ignores their origins. Toward the end of our bird walk at Hammonasset Beach State Park, we got to talking about club-winged manakins. I asked him about their evolutionary history. Prum thinks that long ago, an earlier version of the bird’s courtship dance incidentally produced a feathery susurration. Over time, this sound became highly attractive to females, which pressured males to evolve adaptations that made their rustling feathers louder and more noticeable, culminating in a quick-winged strumming. But why, I asked Prum, would females be attracted to those particular sounds in the first place?

To Prum, it was a question without an answer — and thus a question not worth contemplating. “Not everything,” he said, “has this explicit causal explanation.”

Here Prum simply dismisses something that scientific reviewers mentioned repeatedly—where do female preferences come from? Prum assumes they are random, but there is a thriving field of sexual selection studying female preferences, showing how they might result from natural selection instead of just being “random” (i.e., aspects of neuronal wiring that have nothing to do with natural selection for the preference). Jabr also says, properly, that not all biologists have dismissed the runaway model, as Prum contends they have, but see it as one of a competing panoply of models that are hard to resolve. (Getting this kind of data from nature or even the lab is very difficult, and we weren’t there to see how sexual selection operated in the past.)

But in the rest of the article, Jabr seems to buy a lot of Prum’s contentions without properly evaluating the criticisms of other scientists. For example:

1.) The runaway model is not “Prum’s theory.” This model was first suggested by Ronald Fisher and elaborated and developed by scientists like Russ Lande and Mark Kirkpatrick. Yet Jabr repeatedly refers to the “beauty happens” model as “Prum’s theory”, as when he says that “Prum’s indifference to the ultimate source of aesthetic taste leaves a conspicuous gap in his grand theory.” (That statement is correct except that it’s not Prum’s grand theory.) This misleading attribution of the theory happens repeatedly. Let us be clear: Prum’s book is about presenting, defending, and applying a theory developed by other scientists.

2.) Jabr buys into Prum’s contention that sexual selection is fundamentally different from natural selection. Most biologists, I think, would disagree, seeing sexual selection as a subset of natural selection. That is, sexual selection is a form of selection based on female mate choice rather than other factors. But both sexual and natural selection involve enhancing those traits that affect reproductive success. (Jabr seems to mistake natural selection as a form of selection that enhances survival rather than reproductive success, but in fact the currency of all selection is the number of offspring that survive to spread your genes.). This may seem a semantic question, but both Jabr and Prum use this distinction to suggest that the runaway theory is a big and revolutionary improvement over previous notions of natural selection. This further inflates the runaway theory into something that it’s not.

In fact, natural and sexual selection blend into each other, and in some cases you can’t distinguish them. If a male produces sperm that swim faster than the sperm of other males in his species, and thus he gets more offspring, is this natural or sexual selection? It’s not based on mate choice, but does involve reproductive success. This is a form of male/male competition, analogous to those bull elk who butt horns during mating season, with the winner getting a harem of females. No female choice is involved in either case, but both could be seen as sexual selection. But they also represent natural selection—selection based on some individuals having traits (horns, fighting ability) that enables them to leave more genes.  My own judgment is that sexual selection is simply a subset of natural selection that involves mate choice, and not something fundamentally different.

3.) Jabr leaves out some aspects of Prum’s views that scientific critics have homed in on. Jabr doesn’t mention, for example, that Prum views the runaway model as the “null model” of sexual selection. That is, Prum deems it the model that we should accept unless we have good evidence for other models. But the runaway model isn’t null in that way: it does carry its own assumptions that themselves have to be justified and tested, such as female preference being “random” and not itself initially the result of natural selection or subject to stabilizing selection. The runaway assumes that male traits and female preferences are genetically correlated, and so on. No single model of sexual selection can be regarded as a “null model” to be regarded as a default option in the absence of any evidence.

4.) Jabr doesn’t fairly summarize the extent of scientific criticism of Prum’s book. While he does cite Borgia and Ball’s criticism, he neglects those of Futuyma and especially the thorough paper of Patricelli et al., and thus leaves out some important problems with Prum’s views (see below). Further, Jabr seems to have consulted critics at only the University of Texas at Austin, including my colleagues and friends Gil Rosenthal, Molly Cummings, and Mike Ryan. These people generally work on the sensory bias model of sexual selection, and thus emphasize theories different from Prum’s, but it would have been good to consult others who work on Prum’s model itself. These would include both Mark Kirkpatrick (also UT Austin!) and Russ Lande. I have talked to several “runaway” modelers, and their take is different from Prum’s: while they think the theory can operate, they are wary of its ubiquity in the absence of empirical evidence. This view, by the very proponents of Prum’s favorite model, shows a scientific caution far more admirable than Prum’s dogmatism.

5.) Jabr doesn’t mention at all an important aspect of Prum’s book: Prum’s view that because in some species females have “sexual autonomy” in choosing males, that hearten feminists who, rightfully, are against sexual coercion by human males. This omission by Jabr is a mistake, for this part of Prum’s message is one of its selling points, and surely explains some of the book’s popularity. But we shouldn’t buttress our morals by looking for parallels in nature, for, as I’ve said repeatedly, doing that makes our morality subject to revision via new information about nature. While some moral judgement can depend on empirical information (abortion may be one example), arguments about human rights and autonomy should be independent of how other species behave.

Jabr further ignores Prum’s invidious use of eugenics and comparisons to Nazis and genocide to tar models of sexual selection based on “good genes”. Ball and Borgia explicitly mention this, as do Patricelli et al. in the section of their review called “Birds and bedbugs make bad politics” (all three authors of that review are women).

My view then, is that Jabr’s summary of Prum’s work and the “beauty happens” theory is better than that of any of the summaries in popular venues, but still suffers from a general laziness manifested in contacting only scientists at UT Austin and in failing to summarize much of the criticism leveled by scientists at The Evolution of Beauty. Jabr didn’t do his scientific homework. The definitive popular critique of Prums’s views, as opposed to those that have already appeared in scientific journals, has yet to be written.

The results of sexual selection: male and female Manadrin Ducks (Aix galericulata). Photo from Wikipedia.


Natural selection against clueless cellphone users

November 4, 2018 • 1:45 pm

We’ve all seen people bump into telephone poles and nearly get hit by cars when walking around looking at their cellphones. (Hell, I’ve done it myself, at least with the telephone poles; I never look at a phone while crossing the street.) When I almost bumped into one of these metal poles in Paris, I realized that if they were a little shorter, and had a more injurious top than a simple ball, they could be used to select against heedless cellphone users.

The scenario: someone is walking along and looking at their cellphone, and bumps into a shorter version of one of these poles, say with a metal spike on top. Voilà! Their gamete-producing organs are injured, hurting their fertility. If there’s any genetic variation for using cellphones while oblivious to the external environment, that variation will be reduced by colliding with these “anti-gamete” poles. Within a generation, more people will be using their cellphones responsibly.

Of course this will work only for males, but that’s still selection on half the population, and presumably the genes for obliviousness are expressed in both sexes. Eventually only the nongenetic (socially conditioned) variation will remain.

My hand shows the height that the spikes needs to be; but of course it can be a foot or more in length, dealing with most of the height variation in human males.